Environmental heat responsive circuit interrupter



United States Patent 3,365,559 ENVIRQNMENTAL HEAT RESPONSIVE CliRCUlT INTERRUPTER Ralph L. Sherwood, Westfield, N.J., assignor to Burndy Corporation, a corporation of New York Filed Mar. 4, 1966, Ser. No. 531,967 3 (Ilaims. (Cl. 200-142) ABSTRACT OF THE DISCLGSURE An electrical circuit interrupting device which is intended to operate in response to ambient heat and temperature conditions. Materials capable of absorbing a meltable circuit link are used to assure removal of the molten link material from the circuit path area. Materials resistant to wetting by the molten link material are used as link supports.

This invention relates generally to an electrical circuit interrupter, and more particularly to a device of that nature which specifically eliminates the possibility of inadvertent closing of an interrupted circuit.

A circuit interrupter of this type is especially desirable for use in assuring positive disconnection of a potentially dangerious electrical circuit, particularly under high ambient temperature conditions. The deactivation of an electrically operable weapon system on a crashed or burning plane is one example of conditions which might require such protection. A weapon so protected would run a substantially reduced risk of accidental detonation by the heat generated by a burning plane.

Circuit interrupters of the type commonly used prior to this invention, frequently employed a fusible conductive link, adapted to melt at a certain temperature. Frequently, however, the molten metal of the melted links would run or drip into a shape which would again form a conductive path capable of sustaining current flow.

Accordingly, an object of this invention is the provision of a circuit interrupter in which the material of a temperature-sensitive conductive link is affirmatively withdrawn from a certain non-conductive area, following an initial circuit interruption.

It is a further object of this invention to provide such a circuit interrupter which will respond quickly to an increase in ambient temperature.

In general, the objects of this invention contemplate the provision of a circuit interrupter having the foregoing characteristics which is nevertheless relatively light in weight, easy to manufacture and small in size.

A feature of this invention is the use of a series electrical circuit comprising a pair of plugs of relatively high fusing temperature material having an element of relatively low fusing temperature material interposed between them, wherein the structure of the plugs is adapted to absorb the molten form of the interposed element by capillary action or by alloying of the two materials or by combinations of these mechanisms, upon melting of that element. As a result of this absorption, the interposed element is physically removed from between the two plugs thereby assuring a positive break in electrical continuity of the circuit.

These and other objects, features, and advantages of this invention are more distinctly pointed out in the following specification and claims and will be made more apparent by reference to the accompanying drawings, in which:

FIGURE 1 is a longitudinal cross-sectional view of an interrupter constructed in accordance with this invention, shown before circuit interruption;

FIGURE 2 is a similar view showing the device of FIGURE 1, following a circuit interruption; and

FIGURE 3 is a pictorial representation of the conductive plug which is used in the device shown in FIGURES 1 and 2 to withdraw molten link material from the link area after a circuit path has been interrupted.

Referring now in greater detail to the drawings, the invention may be seen to comprise a housing or body member in the form of tubular or cylindrical container 10, formed of glass or other suitable dielectric material, in which is positioned a fusible element 12 made of a conductive metal or alloy having a sharply defined melting temperature. Representative metals or alloys of this type include indium, and alloys such as Roses alloy and Woods alloy which may include, for example, bismuth, gallium, lead and indium.

The fusible element 12 is positioned relative to cylinder 10 in an aperture 14 within a collar 16. The collar comprises a support means for element 12, preferably formed of a ceramic or other dielectric material which is not subject to wetting by the fusible metal of element 12 in its molten state. Element 12 may be staked or otherwise secured within collar 16 in order to form an integral unit which will facilitate assembly of the complete interrupter.

The integral unit comprising link 12 and collar 16 is positioned axially within the container 10, and a pair of conductivve plugs 18 are positioned on opposite sides thereof in mechanical and electrical contact with the ends of fusible element 12. Plugs 18 comprise cylindrical lengths of a porous or spongy conductive material which is chosen for its ability to rapidly absorb the molten metal of link 12 so as to withdraw it from aperture 14. Such absorbent plugs can be made, for example, by sintering copper particles into an integral mass which is characterized by pores or voids of particular size. Suitable plugs may also be formed by compressing a mass of conductive metal fibers into a given shape in accordance with well known techniques. Regardless of the method of fabrication used, the dimensions of the voids in a plug should preferably be determined relative to the surface-tension characteristics of the molten fusible metal of element 12 so as to preclude dripping or return flow of the metal following its withdrawal into the plug. For material such as indium, a pore size on the order tof microns has been found to be acceptable for this purpose. On the outermost ends of the two absorbing blocks 18 are provided conductive contact plates 20 which include extending terminal portions 22 for establishing electrical circuit connections external to the interrupter.

In constructing the interrupter, pressure may be employed to self-weld the link 12 to the absorbing blocks 18, although it is clear that other joining means may be used. During assembly of the various parts, the two plates 20 may preferably be pressed towards each other within the glass container, so as to assure firm electrical contact between the parts and to achieve the desired overall electrical resistance across the interrupter. A resistance of 100 micro-ohms or less is a typical preferred value for such a device. When sufficient assembly pressure has been applied, the plates 20 may be sealed to container 10 by means of a suitable bonding method such as the formed washer of epoxy material 24.

Indium has been mentioned previously as a desirable material for link 12. This metal is known to have a sharply defined melting temperature of 156.40 C. When this temperature is reached in the environment surrounding an interrupter of this nature, the indium element will melt and be absorbed into the absorbent sponge metal blocks 18 as shown at 12A in FIGURE 2. After opening of the circuit in this manner, the absorbed indium cannot be reunited to close the circuit even under sustained heat, since the molten indium will be held by surface tension within the porous blocks 18. To protecct against arcing problems caused by vaporization of the fusible material due to sustained high ambient temperatures, it is desirable to employ a material having a low vapor pressure, such as indium, which will in turn provide a relatively high arc-over voltage.

With the aid of this description, an interrupter having an overall length of 0.75 inch and a representative diameter of 0.25 inch may readily be produced.

An interrupter constructed in accordance with the foregoing description of this invention, accordingly may be made relatively small and light in weight, and will be characterized by high resistance to shock and vibration effects due, in each case, to the lack of any requirement for moving mechanical parts in its operation.

This invention has thus been described, but it is desired to be understood that it is not confined to the particular forms or usages shown and described, the same being merely illustrative, and that the invention may be carried out in other ways without departing from the spirit of the invention, and therefore, the right is broadly claimed to employ all equivalent instrumentalities coming within the scope of the appendant claims, and by means of which objects of this invention are attained and new results accomplished, as it is obvious that the particular embodiments herein shown and described are only some of the many that can be employed to obtain these objects and accomplish these results.

I claim:

1. A temperature responsive electrical circuit interrupter comprising:

a body member;

a fusible circuit path element, of electrically conductive material having a known melting temperature, mounted to said body member in a given position relative thereto;

a pair of external electrical circuit connection means mounted to said body member and electrically coupled to said fusible element, forming a series electrical circuit through said fusible element;

absorbent means comprising a porous mass of compressed conductive fibers mounted to said body member in contact with said fusible element and forming part of said series electrical circuit between said fusible element and at least one of said pair of external electrical circuit connection means;

said absorbent means operating to withdraw substantially all of the material of said fusible element from said given position upon melting of said fusible element, by absorbing substantially all of the material of said fusible element into said porous mass.

2. The temperature responsive electrical circuit interrupter of claim 1 wherein said absorbent means comprises a pair of spaced apart porous masses having said fusible circuit path element positioned between said porous masses, said interrupter further including support means for said fusible element mounted to said body member between said porous masses and formed of a material characterized by high resistance to wetting by the material of said fusible element in its molten state.

3. A temperature responsive electrical circuit interrupter, comprising:

a substantially tubular housing member of dielectric material;

a fusible circuit path element of conductive material having a known melting temperature, mounted to said housing member in a substantially axially aligned position within said housing member;

support means mounted to said housing member and formed of a material which is resistant to wetting action of the conductive material of said fusible element, said support means supporting said fusible element within said housing member;

a first plug of absorbent conductive material axially disposed within said housing in mechanical and elec trical contact with one end of said fusible circuit path element;

a second plug of absorbent conductive material axially spaced from said first plug within said housing member, in mechanical and electrical contact with the other end of said fusible circuit path element;

closure means sealing the axially opposite ends of said tubular housing member for maintaining a sealed environment within said housing;

first external electrical contact means electrically coupled to said first plug and extending to the exterior of said housing member for establishing external electrical connection to said interrupter; and

second external electrical contact means electrically coupled to said second plug and extending to the exterior of said housing member for establishing external electrical connections to said interrupter;

said absorbent conductive material being porous and characterized by interstitial spaces dimensioned to absorb and retain the material of said fusible element in its molten state so as to withdraw substantially all of such molten material from the axial space between said first and second plugs when said fusible element has melted.

References Cited UNITED STATES PATENTS 1,350,055 8/1920 Ballman 200128 2,017,492 10/ 1935 Glowacki 200131 2,283,656 5/1942 Strom 200-43] BERNARD A. GILHEANY, Primary Examiner.

H. B. GILSON, Assistant Examiner. 

